Support ordering of sub menus

[Hanksha]

What it does

Fixes #7958.
This PR adds the order property to the SubMenuOptions interface used when registering sub menus. The order property is used to sort the sub menus with the menu actions.

Note that this also allows reordering of existing sub menus like this:

menus.registerSubmenu(CommonMenus.File, 'File', {
    order: 'z'
})

How to test

I added a sample file for that called examples/api-samples/src/browser/menu/sample-menu-contribution.ts. This contributes a Sample Menu to the menu bar with order value '2' which should place it right next to the File menu.

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[akosyakov]

code changes look good, tested that sample menu displayed at the proper location

[RomanNikitenko]

@Hanksha @akosyakov
Is it expected to have Sample Menu permanently?
Or it was added for testing goal only?

[Hanksha]

I just added it to @theia/api-samples for testing and to have an example. Is that a problem?

EDIT: Just read this on the readme

The examples here are all deactivated by default. Commands are provided that toggles on the 
example behavior. These commands are prefixed with "API Sample".

I could try to make the example not visible by default but it will make it more complex. Or put the sub menu somewhere less visible.

[RomanNikitenko]

@Hanksha
it's not a problem
At first sight I just thought that it was added for testing goals only.
thank you for clarification!